1. Technical Field
The present disclosure relates to a liquid crystal display having an optical film embedded therein and, more particularly, to a liquid crystal display having a structure in which an optical film for the uniformity and focusing or concentrating light provided by a backlight unit has been laminated on a lower polarizer.
2. Discussion of the Related Art
The application of a liquid crystal display has gradually increased due to its characteristics, such as light weight, a thin profile, and low power consumption. The liquid crystal display is used in portable computers, such as notebook PCs, office automation devices, audio/video devices, and indoor and outdoor advertising displays. A transmission-type liquid crystal display, used in most liquid crystal displays, can display an image by modulating light incident from a backlight unit through control of an electric field applied to a liquid crystal layer.
The backlight unit may be categorized as a direct type and an edge type. The direct-type backlight unit has a structure in which a plurality of light sources is disposed under a liquid crystal display panel. The edge-type backlight unit has a structure in which a light source is disposed to face the side of a light guide plate with a plurality of optical films disposed between a liquid crystal display panel and the light guide plate. In the edge-type backlight unit, the light source radiates light to one side of the light guide plate, and the light guide plate converts a line light source or a dot light source into a surface light source. The edge-type backlight unit has an advantage in that it can be made thinner than the direct-type backlight unit.
A liquid crystal display that includes an edge-type backlight unit according to a related art is described with reference to FIGS. 1 and 2. FIG. 1 is an exploded perspective view showing the structure of the liquid crystal display that includes an edge-type backlight unit according to the related art. FIG. 2 is a cross-sectional view showing the structure of the liquid crystal display that includes an edge-type backlight unit according to the related art, which is taken along line I-I′ of FIG. 1.
With reference to FIGS. 1 and 2, the liquid crystal display according to the related art includes a liquid crystal display panel LCP and an edge-type backlight unit EBLU disposed under the liquid crystal display panel LCP. The liquid crystal display panel LCP has a liquid crystal layer LC formed between an upper glass substrate SU and a lower glass substrate SL, and may be implemented in any liquid crystal mode. The liquid crystal display panel LCP also has an upper polarizer UPOL on the upper glass substrate SU and a lower polarizer LPOL under the lower glass substrate SL.
The edge-type backlight unit EBLU includes a light source LS, a light guide plate LG, and an optical film OPT. The edge-type backlight unit EBLU converts light, output by the light source LS, into a uniform surface light source through the light guide plate LG and the optical film OPT, and provides the converted uniform surface light source to the liquid crystal display panel LCP. Furthermore, a reflection plate REF for returning light that leaks through the bottom of the light guide plate LG to the light guide plate LG may be further provided under the light guide plate LG.
A cover bottom CB is disposed under the reflection plate REF. The cover bottom CB may have a bowl or container shape in which the edge-type backlight unit EBLU is received. Furthermore, the cover bottom CB includes a material having high thermal conductivity and high stiffness so that heat from the light source LS can be smoothly discharged to the outside. For example, the cover bottom CB may be formed using a metal plate, such as aluminum (Al), aluminum nitride (AlN), an electronic galvanized steel sheet (EGI), stainless steel (SUS), Galvalume (SGLC), an aluminized steel sheet (so-called “ALCOSTA”), or a tin plate steel sheet (SPTE). Furthermore, the metal plate may be coated with a high conductivity material for accelerating thermal transfer.
A guide panel GP and the top case TC are disposed at the edge of the liquid crystal display panel LCP. The guide panel GP has a rectangular mold frame in which glass fiber is mixed in synthetic resin, such as polycarbonate. The guide panel GP surrounds the top edge and sides of the liquid crystal display panel LCP and surrounds the sides of the edge-type backlight unit EBLU. The guide panel GP supports the liquid crystal display panel LCP and maintains a uniform spacing between the liquid crystal display panel LCP and the optical film OPT. The top case TC is made of a metal material, such as a zinc plate steel sheet, and has a structure that surrounds the top and sides of the guide panel GP. The top case TC is fixed to at least one of the guide panel GP and the cover bottom CB by a hook or a screw.
A light-emitting device having high brightness with low power consumption, like a light-emitting diode (LED), may be used as the light source LS. The light source LS supplies light to the light guide plate LG. In the edge-type backlight unit EBLU, the light source LS is located at the side of the liquid crystal display panel LCP. That is, the light source LS supplies light to a side of the light guide plate LG in accordance with at least one side of the light guide plate LG.
The light guide plate LG has a panel type rectangular parallelepiped shape having a face corresponding to the area of the liquid crystal display panel LCP. The top surface of the light guide plate LG faces the liquid crystal display panel LCP. The light guide plate LG functions to receive light from the light source LS installed on the side of the light guide plate LG, to diffuse and distribute the light therein so that the light is uniformly distributed within the light guide plate LG, and to guide the light to the top surface on which the liquid crystal display panel LCP has been disposed.
The light guided to the liquid crystal display panel LCP by the light guide plate LG is not suitable for being used as a backlight. For example, the light may not have a uniform brightness distribution over the entire area of the liquid crystal display panel LCP. Alternatively, the light may not have been concentrated in the direction of a viewer with respect to a surface of the liquid crystal display panel LCP. Accordingly, for the light to be used as a backlight, it is necessary to concentrate and diffuse the light in particular locations.
For such a function, the optical film OPT is disposed between the light guide plate LG and the liquid crystal display panel LCP. The structure of the optical films OPT according to a related art is described below with reference to FIGS. 3-4. FIG. 3 is a cross-sectional view showing the structure of optical films, including a diffusion film, in a liquid crystal display according to a related art.
The optical films OPT disposed under the liquid crystal display panel LCP of FIG. 3 have a stack structure that is widely used. For example, the optical films OPT may have a structure in which a lower prism sheet PRL, an upper prism sheet PRU, and a diffusion sheet DIF have been sequentially stacked.
Trigonal prism patterns are disposed in parallel on the top surface of the lower prism sheet PRL. More specifically, a convex peak portion and a concave valley portion are alternately disposed on the lower prism sheet PRL. Pointed peak portions are arranged in parallel in a first direction. The upper prism sheet PRU may also have the same prism pattern as the lower prism sheet PRL. In this case, the tops of the upper prism sheet PRU are disposed in parallel in a second direction orthogonal to the first direction. Light emitted from the light guide plate LG is concentrated in the form of a Gaussian distribution with respect to a normal line for the surface of the liquid crystal display panel LCP, while passing through the lower prism sheet PRL and the upper prism sheet PRU.
The diffusion sheet DIF functions to distribute pieces of light passing through the prism sheets PRL and RPU so that the pieces of light have a uniform brightness distribution over the entire surface of the liquid crystal display panel LCP. For example, in the case of the edge-type backlight unit, a side face in which the light source is positioned may be brighter than a side face opposite the side face in which the light source is positioned. Furthermore, in the case of the direct-type backlight unit, a portion in which the light source is positioned may be brighter than the surrounding portion of the light source. The diffusion sheet DIF functions to uniformly diffuse a brightness distribution of light that is not uniform with respect to the entire surface of the liquid crystal display panel LCP. For such a diffusion function, beads BD may have been distributed on the top surface of the diffusion sheet DIF.
Light becomes suitable for being used as backlight by the prism sheets PRL and RPU and the diffusion sheet DIF, but there may be a problem in that brightness is deteriorated while the light passes through the optical films. This becomes a reason for deteriorating energy efficiency required to generate the backlight. More specifically, brightness is significantly reduced by the diffusion sheet DIF. To solve such a problem, there has been proposed a high brightness diffusion film DBEF. FIG. 4 is a cross-sectional view showing the structure of optical films including a high brightness diffusion film DBEF in a liquid crystal display according to a related art.
The high brightness diffusion film DBEF has a high refraction layer and a low refraction layer stacked thereon, and thus solves a problem in that brightness is reduced by reflecting light lost by reflection to its top surface again. FIG. 4 has the same structure as FIG. 3, except that the high brightness diffusion film DBEF has been disposed instead of the diffusion film DIF.
As described above, the optical films according to a related art have a structure in which they have been sequentially stacked between the liquid crystal display panel LCP and the light guide plate LG. That is, the upper prism sheet PRU is disposed on the lower prism sheet PRL in the lay-down state. Accordingly, a specific air layer is present between the upper prism sheet PRU and the lower prism sheet PRL. The air layer has a refractive index different from that of the prism sheets PRL and RPU, and thus an effect can be obtained in that light passing through the prism sheets PRL and RPU is diffused.
Likewise, the diffusion film DIF or the high brightness diffusion film DBEF is also disposed on the upper prism sheet PRU in the lay-down state. Accordingly, an air layer is present between the upper prism sheet PRU and the diffusion film DIF or between the upper prism sheet PRU and the high brightness diffusion film DBEF. Thus, an effect can be obtained in that light is diffused while passing through the air layers.
However, thickness of the device is increased due to the structure in which the optical films OPT are simply stacked, which becomes an obstacle to the desired thinness of a liquid crystal display. An attempt has been made to make the optical films OPT ultra-thin by laminating them. However, if the optical films OPT are simply laminated, a diffusion effect by using an air layer cannot be obtained because the air layer disappears, resulting in an irregular brightness distribution. Furthermore, a moiré pattern, a rainbow mura pattern, or a pattern of a hot-spot form is generated. Defects, such as irregular brightness, pattern generation, light bleeding failure, etc. are seen as being deficient enough that light cannot be suitably used as backlight, preventing a liquid crystal display from becoming ultra-thin.